Pico-projectors, also known as handheld projectors or mobile projectors or pocket projectors, include miniaturized hardware and associated software for projecting digital images onto a surface. A laser projection system may include a power source (e.g., a battery), a laser light source (e.g., three laser diodes for emitting red, green, and blue light, or RGB beams), and scanning mirrors.
In order to project a colored image, the system includes a digital-to-analog converter or DAC configured to convert a digital signal into an analog one and to drive the laser diodes through respective laser drivers. Mirrors deflect light beams emitted by the laser diodes to a surface pixel-by-pixel, thereby displaying an image.
In order to operate properly, laser scanning projectors undergo a calibration. Such calibration is, e.g., performed to obtain a stable white balance and high resolution of color depth and is to be made during operation of the device. Laser power is a function of the operating temperature, which fluctuates according to the instantaneous laser transmission power.
In general, a laser diode is controlled by a current, as shown in FIG. 1 representing a typical laser characteristic for one color. In first approximation (continuous line), emitted power P is a straight, constant-slope line which is a function of current I, starting from a value (called offset current Ioff). Offset current Ioff is a function of temperature (as indicated by an arrow). In general, the higher the temperature, the higher the driving current supplied the laser diodes to obtain a same power emitted by the laser diode. Dashed line show more accurate characteristics. The offset current Ioff is not exactly defined and the slope tends to deviate from an ideal line, especially at high driving currents.
FIG. 2 shows emitted power vs. current characteristics for typical commercial red laser diodes (with continuous line), green laser diodes (with dotted line) and blue laser diodes (with dash and dot line), wherein current I (on the X axis) is normalized and varies between 0 and 10. Current offset values are not shown in FIG. 2.
In general, the characteristics of FIGS. 1 and 2 also depend on a number of other parameters (such as ageing, manufacturing batches, and so on), so the number of parameters to be controlled depends on the desired accuracy.
Thus, calibration cannot be made once for all at a manufacturing stage, and a dynamic calibration is desired during use.
Laser calibration is usually done using photodiodes. Unlike laser diodes, photodiodes are relatively stable components that could be used to measure the amount of power emitted by the laser diodes. For examples, photodiodes may be arranged on a path of a portion of the emitted light so as to measure the correspondent emitted power value. The measured emitted power value may then be compared with the nominal power value. The thus obtained error may then be used for calibration.
Calibration may be made using a calibration photodiode for each laser diode. This uses three photodiodes which are isolated from each other and from external light, so as to operate properly. As a consequence, this involves high costs due to the need of the additional components and their packages; in addition it is very cumbersome. Therefore, this is not suitable for pico-projectors, wherein costs and dimensions are critical parameters.
An alternative, cheaper implementation is to have a single calibration photodiode that detects a mixture of the light emitted by each of the laser diodes. However, with this, since calibration is made during normal operation, the amount of the three RGB colors varies according to the projected image and the calibration photodiode provides a mixture power, whose composition is not exactly known. Thus, it is not a simple task to calibrate each monochromatic laser diode, in particular without interfering with the displayed image, as explained below.
The measure is generally made in suitable points of the image. For example, the measure may be made at special points purposely projected outside the image (see, e.g., point A in frame F outside image 100 in FIG. 3). However, it is complicated with current pico-projectors to hide the calibration points. In fact, a typical exit window of a typical pico-projector is of a range of 10 mm×7 mm, while the spot diameter at the exit is typically of a range of 1 mm. To block an entire spot it would be thus necessary to divert the ray at angles extremely over the usual operating angles. As a result, it is very difficult to hide such calibration points, without blocking the image, which is undesired.